gentamicin won't work, it's close to geneticin but isn't toxic to mammalian cells whereas geneticin is, so if you want to use it for seletcion, you want to use geneticin instead of gentamicin.

-vairus-

try zeocin

-scolix-

by relative preference order :puro hygro G418/zeo these are only the one i've used.but i know that Blasticidin Phleomycin (for mmore "resistant" Zeo clones)try visiting cayla's website for some references on antibiotics.